Application of domain inversion to electro-optic modulators is reviewed for the cases of the integrated-optic directional coupler and Mach-Zehnder interferometer. Implementation of domain-inverted sections in these devices is shown to result in distinct advantages over the conventional counterparts.

A new class of oscillators based on photonic devices is presented. These opto-electronic oscillators (OEO's) generate microwave oscillation by converting continuous energy from a light source using a feedback circuit which includes a delay element, an electro-optic switch, and a photodetector. Different configurations of OEO's are presented, each of which may be applied to a particular application requiring ultra-high performance, or low cost and small size.

The dynamic range ofwideband microwave externally modulated fiber optic transmission systems is limited by the power handling ability ofthe photodetectors used in the receivers. External modulators can be designed to reduce the optical carrier of the output by using a branch structure to split offsome ofthe input optical carrier, and coherently reduce the optical carrier at the output. A number ofdesign issues relating to modulator fabrication and bias point control result from these approaches.
Keywords: optical carrier suppression, external modulator, SFDR, photodiode power handling, fiber optic links

Planar electroabsorption InP/InGaAsP waveguide modulators suitable for RF applications have been fabricated using the photoelastic effect. The planar device structure is achieved by using WNi thin film surface stressors for lateral waveguiding and helium implantation for electrical isolation between devices. These are the first reported frequency measurements on a photoelastic InP/InGaAsP waveguide modulator.

Large spurious-free dynamic range (SFDR) has been measured in a fiber optic link using an InGaAsP electroabsorption (EA) waveguide modulator. Link phase noise is investigated and conversion of AM noise to phase noise appears to be an issue in the EA device. Preliminary measurements of the EA link show inferior close-to-carrier phase noise compared to a link using a Mach-Zehnder modulator with similar SFDR. Optical feedback from the output coupling fiber is shown to contribute partially to close-in phase noise.

The interwell coupling effects on the small-signal response of unstrained multiple quantum well (MQW) lasers have been studied theoretically. However, in the previous study the transition matrix element of an isolated QW was assumed. The rate equations are solved again for the intensity modulation response but including a matrix element of a coupled double QW (DQW), and then the results are compared with the previous ones to confirm that the former study assumption, referred above, does not make any significant qualitative difference for the example that we have been considering; and also, as it is advantageous to reduce computational time, it was used to calculate the frequency and damping rate of the relaxation oscillations. Results and discussion for these characteristics of a GaInAsP-InP DQW laser are presented as a specific example. We conclude that, in contrast to the common belief, the coupling effect can enhance the modulation characteristics under certain conditions.

The coupling efficiency between laser diode arrays or OEIC components and single-mode fiber ribbons drops rapidly with increasing misalignment tolerance. There are trade-offs between the allowed minimum coupled power and the cost of establishing the required alignment. We demonstrate how tapered polymer waveguides may be used to match the laser mode to the fiber mode, resulting in a module with decreased alignment requirements for a given coupling efficiency. Waveguides with tapered mode profiles have been constructed using photobleaching of a guest/host dye/polymer system. Amoco 4212 polyimide doped with DCM dye was chosen as the waveguide material due to its good thermal stability, and its simple processing which allows multilayer waveguides to be readily fabricated. In particular, waveguides which simultaneously taper the mode both laterally and vertically have been designed. This waveguide system is compatible with assembly of the laser and waveguide components into a module by flip-chip soldering. Our solder self-alignment technique achieves accurate alignment of touching chips through an understanding of the dynamics during bonding.

Injection locking of a semiconductor laser can induce major changes in the modulation characteristics of the laser. A small signal analysis using the lumped-element model shows that both the frequency and damping of the characteristic resonances of the coupled complex field and free carriers (gain medium) are modified. The detuning between the injected field and the free-running oscillating field, the amplitude of the injection field relative to the free- running field, the linewidth enhancement factor, the cavity photon and spontaneous carrier decay rates, and the field enhancement of the decay rate are all key parameters in determining the changes to the modulation characteristics. For a broad range of parameters, there is simultaneous enhancement of the modulation bandwidth and stable, locked operation. The enhancement requires that the frequency of the locking field be detuned from the injection-modified frequency of the cavity resonance. This causes a resonant enhancement of the modulation sideband associated with the preferred frequency of the optical cavity. Bandwidth enhancements beyond the free-running laser limit are possible over a range of injection levels and injection frequency detunings.

Efficient InAsP/GaInP multiple-quantum-well electroabsorption waveguide modulators have been developed at 1.3 micrometers wavelength. The modulators exhibit a typical fiber-to-fiber optical insertion loss of 7 dB, a slope efficiency of 1.0 V-1, and an optical saturation intensity in excess of 10 mW. The 3 dB electrical bandwidth is dictated by the device capacitance and a 13 GHz bandwidth has been measured for a 3 micrometers wide 180 micrometers long modulator. In a microwave photonic link without amplification, a RF link gain as high as -31 dB is achieved at 0.9 mA photodetector current and 1 GHz subcarrier modulation frequency.

Waveguide diode ring lasers (WDRL) in a triangular geometry exhibit a unique combination of characteristics including low-noise, medium power output, small size, good efficiency and low cost integrability and manufacturability which make them attractive candidates for analog optical links requiring large dynamic range and bandwidth. We show that 10 - 20 mw low noise (< -150 dBm) lasers appear feasible which provide low cost manufacturability and integrability. With integrated small amplifiers 85 mW single mode output has been shown. Because these WDRLs operate with unidirectional traveling waves in entirely single mode waveguide and contain a natural output coupler, they show a combination of attractive features not found in other lasers.

To implement millimeter wave photonic links using high speed optical modulators, RF input efficiency to the modulator is an important consideration. In this paper we discuss design and fabrication of ultra high speed multiple quantum well electro-absorption modulators for narrow band applications up to 40 GHz. In order to obtain higher RF efficiency at working frequencies of 20 GHz, 25 GHz and 40 GHz, modulators with monolithically integrated matching circuits were designed and fabricated utilizing co-planar waveguide MMIC technologies. Measurement results show excellent matching at specific frequencies with S11 of -16 dB for the 20 GHz devices, -20 dB for the 25 GHz devices and -36 dB for the 40 GHz devices. At least 6 dB of improvement on optical modulation efficiency can be expected over modulators without impedance matching.

The rf performance of fiber optic links is often limited by the maximum optical power rating of the photodiode receiver, particularly in short distance, point to point applications where the received power can be rather high. Required attenuation of the optical power can significantly compromise performance parameters such as the gain, noise, and dynamic range of the link, as well as adding extra cost and complexity in installing and maintaining the link. This paper presents results from the development of a high speed InGaAs photodiode (16 GHz) which can operate at a dc photocurrent as high as 15 mA, or 20 mW of received optical power.

A four-layer asymmetric waveguide structure using a nonabsorbing 1.08 eV bandgap InGaAsP waveguiding layer has been studied for high saturation power, high speed waveguide photodiode operating at 1.32 micrometers wavelength. A peak photocurrent of 32 mA corresponding to an optical power of 76 mW has been obtained for 40 GHz waveguide photodiode.

We propose a new microwave photodetector with high responsivity, which can handle both high optical power and deliver high output microwave power. The distributed absorption waveguide photodiode (DWGPD) is specially designed to distribute equally the photocarriers over the maximum surface area available, in order to reduce the non linear effects of electrical response due to electric field screening effects. The expected calculated performances of this DWGPD is compared to other type of broadband photodetectors in terms of linear microwave output power, quantum efficiency and thermal behavior. The first fabricated DWGPDs have a responsivity of 1 A/Watt at 1532 nm, linear response up to 10 mA and cutoff frequency of 18 GHz.

Optoelectronic receiver arrays have been developed for analog radio-frequency photonic links. The monolithic optoelectronic integrated circuits contain photodetectors and transimpedance preamplifiers that are based on InGaAs/InAlAs/InP heterojunction bipolar transistors. These receivers also contain on-chip DC-blocking filters and impedance-matching circuits, as needed for analog applications. Some receivers accommodate signals greater than 18 GHz and others have effective responsivities greater than 11 A/W. Receivers having separate chips of photodetector arrays and transimpedance amplifier arrays have also been developed. The photodetector arrays are passively aligned to optical fibers by means of silicon carriers.

We studied ultrafast transport dynamics of ultrawide-band surface-normal p-i-n photodetectors under high illumination and high field using electro-optic sampling techniques. Under reverse bias and high illumination, saturation nonlinearities are dominated by the collapse of the electric field from space charge screening. A transient forward external current is observed with a duration less than 4 ps. The external bias required to compensate these saturation effects increases with increased illumination.

We describe the design of single frequency array transmitters and their application in RF-photonic systems. In addition, we present an array-based packaging technology that is based on passive-alignment with Si-waferboards.

A reflection type volume hologram is made inside a photorefractive single crystal fiber. The volume hologram is generated by interfering counter-propagating beams in the fiber. Since the volume hologram is very sensitive to wavelength, the hologram can be used as a narrowband filter. The filter can be tuned using the thermal or electrooptic effect of the photorefractive fiber.

Microwave optical splitter/amplifier integrated chip devices using semiconductor optical amplifiers are investigated for use in large optical signal distribution systems requiring high dynamic range. After demonstrating the need for optical amplification in large optical distribution systems, we show that the amplifier length should be minimized in order to minimize the noise figure, and that the optical power level should be well below the saturation power. We have demonstrated 11 dB fiber-to-fiber RF gain in a 1 X 4 device, and 15 dB single-pass optical gain in an active 1 X 10 multimode splitter.

A fiber-optic approach for low-loss true time delay of wideband RF signals for phased-array-antenna beamsteering is presented. An optical carrier modulated by the RF signal of interest is launched into a delay-line fiber composed of optical Bragg reflection gratings written holographically into the core of a single-mode fiber. The desired beam steering is realized by tuning the optical carrier wavelength for reflection from the appropriate grating. Radiation testing of various fibers with Bragg gratings has been performed indicating preferable fiber types. True time delay offers much reduced beam squint and sharper antenna superior nulling compared to phase shift scanned antenna. Examples of applications of this concept showing its advantages are presented.

Free space optical systems are described that use switchable grating technology and nonlinear materials to form digital time shifters. A cascade of n independently controlled gratings provides 2n evenly spaced time delay paths. Featured characteristics including potential for excellent switch isolation, spurious beam and crosstalk suppression, reduction in complexity, and low insertion loss are discussed in the context of phased array applications. Varied configurations are discussed including hybrid free- space/guided wave configurations for long time delays; transmission and double-pass digital optical time shifters; and entire array drivers that replace many single-channel time shifters with a single configuration. Using free space micro-optics, many independent time shifter configurations can be compactly stacked. A novel noise suppression device is discussed that enhances the channel isolation and signal purity of the systems.

A wavelength-selective photonic time delay filter is proposed and demonstrated. The device consists of an optical phased-array waveguide grating in a recirculating feedback configuration. It can function as a true-time-delay generator for squint-free beam steering in optically- controlled phased-array antennas. As the photonic filter uses the optical carrier wavelength to select the desired time delay, a one-to-one map is established between the optical carrier wavelength and the desired antenna direction, thus eliminating complex switching networks required to select the appropriate delay line. The proposed device can also function as the encoder/decoder in wavelength-CDMA. The concept uses a waveguide prism in a symmetric feedback (recirculating) configuration. The modulated optical carrier is steered by the waveguide prism to the appropriate integrated delay line depending on the carrier wavelength. The signal is delayed and is fed back into the symmetric input port. The prism then focuses the delayed beam into the common output port. Thus three sequential operations are performed: (1) wavelength demultiplexing, (2) time delay, and (3) wavelength multiplexing. It is important to note that the recirculating photonic filter has no 1/N loss; all the power at a given wavelength is diffracted into the output port. Furthermore, high resolution (6 - 8 bits) can be obtained in a compact integrated device. A prototype regular recirculating photonic filter true-time delay device was realized using a 8-channel arrayed-waveguide grating demultiplexer and external (off-chip) fiber delay lines. The grating was fabricated in the silica waveguide technology with 0.8 nm channel spacing (FSR equals 6.4 nm) and operating in the 1.5 micrometers wavelength range. Light from an external cavity tunable laser was rf modulated at 10 - 40 MHz and was coupled into the arrayed waveguide grating chip and time/phase measurements were performed sing a digital oscilloscope. Feedback delay lines consisted of optical fibers of different lengths connected between 4 symmetric pairs of input and output ports. The results clearly demonstrate the Recirculating Photonic Filter's ability to perform wavelength-selective true-time delay. Furthermore, as expected, the delay has the desired property of being independent of rf frequency.

Photonic programmable binary delay lines capable of long total delay with small incremental intervals can greatly simplify delay line networks for phased array systems. With the increase in operation frequency and angular scan resolution, the delay length accuracy can reach magnitudes of micrometer for millimeter wave frequencies, while the largest delay length can be tens of centimeters for even a moderate number of array elements. Using micro-fabrication technologies, an integrated optic delay line/switch network can achieve such delay performance on a single integrated optic chip that is compact and light weight, and of low optical insertion loss. Polymer nonlinear optical materials have unique features that are particularly attractive to waveguide photonic integrated delay line applications. Long polymer waveguides can be fabricated on a large inexpensive wafer with microelectronics resolution, and integrated with fast electro-optic switches and broadband electro-optic modulators. With the advances in materials synthesis and device fabrication, such low cost polymer programmable delay line chips become increasingly realistic. We propose here an integrated polymer delay line design and report fabrication of polymeric photonic switches and modulators. The related technical issues, including material stability issues, that may affect device design, fabrication, and applications are also discussed.

This paper describes a bias-free, high-dynamic range, phase- modulated fiber optic link. An optical delay line filter is used for both phase demodulation and optical carrier suppression. A spur free dynamic range of 114 dB-Hz2/3 is experimentally demonstrated at a frequency of 12.5 GHz.

A new photonics concept for true time delay beamforming and steering is described. To delay a signal an array of fibers, or other optical waveguides, containing cavities of different resonant frequencies are used to channelize the signal. Each spectral component of the signal is phase shifted by an amount proportional to the frequency of that spectral component and proportional to the time delay desired. These phase shifted spectral components are then summed to obtain the delayed signal. This new approach does not rely on switching between different lengths of delay lines. As a result, the pointing direction of an antenna array can be finely controlled over a continuum of angles, and the time to change direction can be on the order of 10 nanoseconds or faster. The concept has been refined, analyzed, and implementation issues addressed. The approach appears to be feasible. All the integral components with characteristics necessary to meet the requirements of an operational true time delay beamforming system are realizable. The time delay module as a channelizer has a number of alternate applications. For electronic warfare the channelizer provides the ability to analyze a wide bandwidth signal on the order of tens of Gigahertz at a resolution of 10 to 20 Megahertz. As a wavelength demultiplexer in optical communications, it would allow for a greatly increased density of channels within a given spectral range.

In this paper, we discuss the applications and also several important system issues: insertion loss, noise figure, dynamic range and cost relating to photonics for wideband phased array antennas. This discussion is based on the work that we did on an L-band Optical Control of Phased Array Project funded by DARPA/Rome Lab. The antenna has been delivered to Rome Lab for further demonstration.

We have developed a novel opto-electronic transceiver designed to provide a jam-resistant, high-security, low- power rf communications link. By employing picosecond photoconductivity and a time-hopped spread-spectrum architecture, the transceiver can realize multi-octave instantaneous rf bandwidth. The design is capable of processing gains well above those of existing spread- spectrum communications systems. Hence, significant jam resistance is achievable with a minimum of rf power. The transceiver is particularly well suited to serve as the physical layer of a packet radio system and can be packaged in a hand-held unit with Q-switched diode laser technology. We present results from an opto-electronic digital spread- spectrum data link that demonstrate some of the capabilities of this approach.

This paper describes a unique, linear, sensitive, optically triggered RF switch for use in antenna switching, antenna reconfiguration, and microwave circuit control. California Microwave, Inc. is actively seeking practical implementations of such switches for several applications, most notably our Synaptic antenna, and have trademarked the devices as PhotistorsTM. Three key technologies are necessary for the successful implementation of Synaptic antennas: PhotistorsTM, optical sources, and digital control. The scope of this paper will be limited to PhotistorsTM and their application. Steve Wojtczuk of Spire Corp. has been responsible for the design and fabrication of the current iteration of these devices. The following topics will be presented: (1) an introduction to this technology, (2) details of the switches, (3) current applications, and (4) future development.

A new type of RF-photonic antenna design which uses an optical serially-fed phased array is proposed for applications in radar and communication systems. This antenna has the advantages of true time delay and yet only requires one tunable laser and one fiber optic grating delay for beam steering. In addition to discussing the system operation in transmit mode, we also present initial experiments establishing the viability of the basic serial-feed design approach by experimentally demonstrating the concept of optically controlled directional video broadcast using a 9 GHz sub-carrier. Our video transmission results are the first use of an optically controlled serially-fed phased array antenna as a communication device.
KEYWORDS: true-time delay, fiber Bragg gratings, phased-array, video broadcast, photonic antenna, wireless communication

Applications oflong fiber optic delay lines as a radar test target unit to improve radar testing are discussed. The unit intercepts a small amount ofthe radar's transmitted energy, delays it in a fiber optic delay line, and then feeds it back into the radar as a target return. Both radar system operation and use ofthe test target in end-to-end radar testing and RF unit fault isolation are also described.
Keywords: fiber optics, delay lines, radar testing, test target

In-building radio propagation measurements at 900MHz are used to analyze the dynamic range requirements and optimal architecture for a distributed antenna network. The ideal performance/cost ratio for the network is found to be achieved with a low-cost hybrid fiber-coax architecture. A system design procedure and field-trial results are presented.

We introduce a multichannel RF correlator that is capable of simultaneously computing the correlations between a given input signal and a set of stored reference waveforms. The system relies on angularly multiplexed volume holograms to store the reference waveforms and is a new example of an emerging class of RF systems that use photonics to solve complex problems. Preliminary experimental data derived from a demonstration system are presented and discussed.

A photonic radio-frequency synthesizer is proposed in which two or more distributed-feedback laser diodes are injection- locked to different optical modes of an actively-mode-locked external-cavity laser diode. The injection-locked laser outputs are combined to produce a high-stability, low-phase- noise heterodyne optical signal with frequency separation in the 1 GHz through millimeter-wave frequency range that is a multiple of the active mode-locking reference oscillator frequency. Since the phase fluctuations of the individual longitudinal modes of the mode-locked laser are highly correlated, the phase fluctuations of the heterodyne output signal are minimized. This approach yields optical heterodyne signals with widely tunable millimeter-wave frequency separations and low phase noise, without the need for a wideband millimeter-wave electronic reference oscillator and frequency-locking control loop as required in previous heterodyne signal generation schemes.

We propose and demonstrate a tunable photonic radio frequency notch filter. The filter has a single delay line architecture and incorporates a new wavelength-selective true-time-delay. The time delay in the feed-forward loop can be varied by simply changing the optical wavelength resulting in a shift of the null of the filter response. Application in radar systems is discussed.

The development of an optical based RF architecture will benefit modern avionics, collection, and radar systems. Work to date in this field has led to the development of the current transmission line components (mach zehnder modulators, optical detectors, and fiber optic line), but the performance of these approaches is less than stellar. What is needed is a roadmap to develop optical components that can work together to replace entire systems with more functionality for the size, weight, and cost of typical RF approaches.